Here's a run down of what I did; hopefully it can help out someone else.
I started by asking all the kids to look around and name all the things they saw about each other that was the same (all have noses, eyes, hair, shoes, etc). Then I asked them to look around and tell me some of the things that were different (hair color/length, size, clothes, eye color, etc). Then I told them I was going to read them a story about two little kids that were best friends. And I asked them to listen for all the things that were the same about the kids, and all the things that were different. Then I read My Friend Isabelle by Eliza Woloson (a great book about a little boy who is normal developing and a little girl with Down syndrome).
When the book was finished we talked about some of the differences between the two kids. Then I told them that like the little girl in the book, Antalya was born with something called Down syndrome. I didn't get too technical, but just mentioned that inside all of our bodies are chromosomes that tell our body what color hair we will have, what things we will be good at, what color our eyes will be, etc. Most people are born with 46 chromosomes, but Antalya was born with 47. Because she has one more, it makes some things easier for her, and some things a lot harder for her.
I talked to them about some of the things that are really easy for Antalya (she's flexible, she's really good at sharing, she's good at showing love and is a really good friend, and I threw in the fact that she is better on computers and iPhones then most adults). Then I had the class tell me some of the things they have noticed that are really hard for Antalya. Some of the things that came up were that it's hard for her to keep her shoes on :), it's hard for her to keep her hands to herself, it's hard for her to write, and that it is really hard to understand what she says. We talked about what to do if she starts playing with their hair, bugging them, or just won't keep her hands to herself (One sweet girl raised her hand and said Antalya likes to play her hair, I reminded her that she can tell the teacher if Antalya doesn't stop. But she quickly said, "no, I like when she does!").
Then I had all the kids put a big marshmallow in their mouth and try to tell me what they ate for breakfast. They thought it was hilarious that they couldn't understand each other. I explained that that is what it is like for Antalya - because her tongue is bigger, it's like she's always trying to talk with a marshmallow in her mouth.
We ended with talking about some of the things they could do to help Antalya. Then I gave all the kids a coloring book to take home so they could tell their families about Down syndrome (I didn't use all the pages; I took out the ones I thought weren't necessary). Overall, it went really good, and I was really glad that I did it.